Predistortion compensation amplifiers (PD compensation amplifiers) are used as high-frequency distortion compensation amplifiers in, for example, base station apparatuses of mobile communication systems and the like to compensate for distortion produced by an amplifier that amplifies signals that are the object of wireless transmission to mobile station apparatuses and the like.
FIG. 4 shows an example of a basic configuration of an adaptive PD distortion compensation amplifier (APD distortion compensation amplifier).
The APD distortion compensation amplifier shown in the drawing comprises a delayer 61, a predistortion circuit 62, amplifier 63 that is the object of the distortion compensation, an envelope detector 64, a memory 65 for storing compensation tables, a sideband power detector 66, an integrator 67 and a controller 68. The predistortion circuit 62 is comprised of a variable attenuator 71 and a phase shifter 72. Stored in the memory 65 as compensation tables are a variable attenuator table 73 and a phase shifter table 74.
An example of the operations carried out by the illustrated APD distortion compensation amplifier will now be described.
Upon a high-frequency input signal being input to the delayer 61, the envelope of the input signal is detected by the envelope detector 64. After the amplitude of the output from the delayer 61 is adjusted by the variable attenuator 71 and the phase is adjusted by the phase shifter 72 in the predistortion circuit 62, it is input to the amplifier 63. In the amplifier 63, the distortion component produced by the predistortion circuit 62 cancels the distortion component produced by the amplifier 63 with respect to the input signal, and the distortion-compensated high-frequency transmission signal is output from the amplifier 63.
The accuracy of the distortion component cancelling is determined by how good the correspondence is between the distortion characteristic of the predistortion circuit 62 and the nonlinearity of the amplifier 63. The correspondence between envelope information on input signals and variable attenuator 71 control modes (compensation data) is stored in the variable attenuator table 73, and the correspondence between envelope information on input signals and phase shifter 72 control modes (compensation data) is stored in the phase shifter table 74. Input signal envelope information obtained by the envelope detector 64 is checked with the variable attenuator table 73 and phase shifter table 74 of memory 65, and the compensation data thus associated with the envelope information as it changes from instant to instant is read out from each of the tables 73 and 74, and the amount of attenuation of the signal amplitude by the variable attenuator 62 and the amount of change in the signal phase (amount of phase shift) by the phase shifter 72 are controlled.
Feedback control is used to improve and optimize the accuracy of the compensation amount. That is, in the power spectrum of the transmission output from the amplifier 63, sideband component power constituting the adjacent channel power is detected by the sideband power detector 66, the detection results are integrated at fixed time periods and the integration results input to the controller 68, and the values stored in the variable attenuator table 73 and the phase shifter table 74 are updated, based on the integration values input to the controller 68. Thereby, optimal compensation data signals are adaptively controlled to be input from the variable attenuator table 73 and phase shifter table 74 to the variable attenuator 71 and phase shifter 72 of the predistortion circuit 62.
With respect to input signal timing, because a delay time arises in the timing of control processing by the predistortion circuit 62 in accordance with envelope information transmission output sideband power information, as described above, before input to the predistortion circuit 62, the delayer 61 delays the input signal by an amount of time that is the same as the delay time required for the control processing.
An example of a conventional technology relating to a distortion detector that detects distortion produced by an amplifier will now be described.
The prior art includes a distortion detector (hereinafter referred to as distortion detector A) that, when the amplifier amplifies a plurality of fundamental waves, uses a narrow band filter to detect third order intermodulation distortion. With the distortion detector A, it is difficult to detect third order intermodulation distortion using a filter when, for example, the frequency of the plurality of fundamental waves is not fixed (see, for example, Patent Reference 1).
The prior art also includes a distortion detector (hereinafter referred to as distortion detector B) that, when a digital modulated wave is amplified by a main amplifier, a filter connected in parallel to the output terminal of the main amplifier is used to transmit waves that are multiples of the signal frequency, the transmitted signals are amplified and a wave detector is used to detect the amplified signals and convert them into direct-current voltage signals corresponding to the input level, detects the Adjacent Channel Power (ACP) produced by the nonlinearity of the main amplifier on the basis of the direct-current voltage signals (see, for example, Patent Reference 1).
Here, the distortion detector B is detecting the ACP as the distortion component produced by the amplifier. It is suitable for ACP detection since, compared to a double wave, the amount of power change in a distorted triple wave is large. As one example, the filter of the distortion detector B was made to transmit a double wave or triple wave (see, for example, Patent Reference 1).
Patent Reference 1
Unexamined Japanese Patent Publication 2000-286644
However, with a prior art PD distortion compensation amplifier such as, for example, the one shown in FIG. 4, if the distortion component should greatly increase due to an anomaly in the amplifier 63, it becomes impossible for the distortion component generated by the predistortion circuit 62 to cancel the distortion component produced by the amplifier 63. When this situation arises, it is necessary to expeditiously stop transmission, since the distorted output becomes spurious and interferes with other radio waves. However, then there is the problem that when transmission is stopped, the PD distortion compensation amplifier is unable to operate.
The present invention was accomplished to resolve such problems in the prior art, and has as its object to provide an amplifier apparatus that, when compensating for distortion produced by an amplifier in the course of amplifying a signal, can continue to operate even when, for example, the amplitude of the distortion component contained in the amplified signal after distortion compensation becomes large.